Conventional transistors typically use a same material for the source and drain. The materials that are used are customized for a specific application. For example, for applications where significant power requirements are needed from a transistor, a transistor material having a high breakdown voltage is desirable. Such materials include those materials known to have a high bandgap energy. Currently asymmetric transistors provide advantages that improve transistor device performance. In addition, for transistors that require a high breakdown voltage and low drain junction current leakage a high bandgap material in the drain region is desirable.
Another design parameter for transistors is the consideration of the amount of transistor channel strain. It is desired to have as high a channel strain in a transistor as possible. In order to maximize the channel strain, a high strain material is used in both the source and drain. However, known high strain materials have a low bandgap energy and therefore lower the transistor's breakdown voltage and create higher drain junction current leakage. Thus, the design of transistors involves a tradeoff to maximize two desired properties that cannot be found in a single material.
Others have proposed asymmetric transistor structures with the use of asymmetric dopant implants. The objectives in using asymmetric transistor structures are to minimize the impact of junction capacitance on circuit performance, reduce junction current leakage and gate current leakage.
Transistors that are doped asymmetrically do not address the breakdown and junction leakage that arises from the reduced bandgap energy of the material in the source and drain of the transistor.
Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve the understanding of the embodiments of the present invention.